This invention relates generally to semiconductor transistors. In particular, the invention relates to heterojunction bipolar transistors. Heterojunction bipolar transistors (HBTs) offer much higher speed of operation than the more prevalent metal-oxide-semiconductor field-effect transistors (MOSFETs) or even conventional homojunction bipolar transistors, e.g., pnp or npn silicon transistors. Because HBTs offer high speed, a high current driving capability, and a low l/f noise levels, HBTs are becoming popular for use as integrated switching devices and microwave devices in wireless communications systems and sub-systems, satellite broadcast systems, automobile collision avoidance systems, global positioning systems, and other high-frequency applications. One application in which HBT use is increasing is in the design and manufacture of wireless electronic devices, such as wireless telephones and other like electronic devices that are capable of communicating with a network in a wireless manner.
Although HBT""s offer many benefits over bipolar silicon transistors, there remains a need to improve or extend the frequency response of a GaAs based HBT. One manner to extend the frequency response of a GaAs based HBT is to establish a gradual change in bandgap across the base layer of the HBT. The bandgap shift establishes a conduction band energy gradient that constitutes a quasi-electric field that drives electrons across the base layer by drift and by diffusion. As such, the amount of time necessary for electrons to traverse the base layer is significantly reduced. Moreover, the graded base layer operates to minimize the electron transit time in the base region thus, increasing the frequency at which the HBT incremental current gain drops to unity or often referred to as the current-gain cutoff frequency (fT).
The beneficial effects of base grading have been demonstrated in AlGaAs and InGaAs base layer grading. Unfortunately, the aluminum in the AlGaAs device demonstrates a high affinity for atmospheric oxygen. The oxygen, especially at a heterostructure interface, tends to degrade electrical properties of the device over time. The degraded electrical properties are often manifested in reduced mobilities and carrier trapping. Moreover, it has also been demonstrated that the indium material in the InGaAs HBT has several undesirable properties. The amount of indium that can be incorporated into a graded base layer is limited due limitations in the critical layer thickness. The addition of indium tends to decrease the acceptor impurity incorporation, which is not desirable.
The present invention provides a graded base GaAs HBT having an increased or extended frequency response that addresses the above-described problems associated with graded base AlGaAs and InGaAs HBT devices. This is accomplished by the introduction of antimony (Sb) in a graded base layer of a GaAs-based HBT.
The heterojunction bipolar transistor of the present invention includes a collector region having at least one layer disposed on a substrate to form a first stack, a graded base region having at least one layer disposed on a portion of the collector region to form a second stack. The HBT further includes emitter region having at least one layer disposed over a portion of the graded base region to form a third stack and a contact region having at least one layer disposed over a portion of the emitter region to form a fourth stack. The graded base layer is doped with an impurity concentration that gradually increases from a first surface of the graded base layer adjacent to a first layer of the first stack to a second surface of the graded base layer adjacent to a layer of the third stack within the heterojunction bipolar transistor. The grading and the doping of the base layer with a high concentration of impurities results in a reduction of a base resistance value for the HBT, which, in turn, improves or extends the current gain cutoff frequency (fT) to about 100 GHz of the HBT.
The present invention also provides a method for forming a compound semiconductor device having an extended frequency response. The method provides for forming on a substrate a collector region having at least one layer to form to a first stack and forming a base region having at least a graded base layer on a portion of the collector region to form a second stack. The method further provides for forming an emitter region having at least one layer on a portion of the base region to form a third stack, and forming a contact region having at least one layer on a portion of the emitter region to form a fourth stack. The forming of the graded base layer allows the fabricated compound semiconductor device to realize a quasi-electric field to reduce the base transit time of electrons. The method further provides for doping of the graded base layer in a high concentration of impurities to reduce a resistance value associated with the graded base layer to improve or extend the fT of the device to about 100 GHz.